The etiologies of dental caries and periodontal diseases are closely associated with dental plaque, the biofilm that develops from microbial colonization of the tooth surface. Colonization is initiated by a limited number of gram positive species, primarily different viridans streptococci and actinomyces which typically occur in vivo as members of a complex microbial community. The formation of this community depends on an array of specific adhesive interactions which includes those detected by in vitro coaggregation between different bacteria. Interactions of this type are mediated by GalNAc- or Gal-sensitive adhesins on certain strains actinomyces or streptococci and complementary receptors on other strains of at least four viridans streptococcal species. The adhesins of actinomyces are associated with the type 2 fimbriae of these bacteria and the coaggregation receptors of viridans streptococci with a family of structurally related cell wall polysaccharides. Recognition of these polysaccharides as receptors depends on the presence of a host-like disaccharide motif, either Gal'163GalNAc or GalNAc'163Gal, within the oligosaccharide repeating unit of each polysaccharide. In contrast, the antigenic properties of these polysaccharides are more closely correlated with the non host-like features of these molecules. Current studies are directed toward the identification and characterization of genes for receptor polysaccharide biosynthesis. These genes were detected by Southern blotting of streptococcal genomic DNA with probes prepared by PCR of genes for the 19F capsular polysaccharide of Streptococcus pneumonia. In addition, a fragment of S. gordonii genomic DNA that contains genes for L-rhamnose biosynthesis was cloned and is being characterized. Information gained from these studies should provide a basis for the development of molecular approaches to assess the recognition role of streptococcal receptor polysaccharides in microbial colonization and biofilm formation and hopefully, will also contribute to the development of new approaches for the prevention of plaque-related diseases.